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Gold Mining in the Judith Mountains, Montana
Mining and Scientific Press June 10, 1916
By O. W. Freeman
INTRODUCTION. On the plains of Montana east of the continental divide rise several detached
smaller mountain groups. Mineral deposits are found in most of them. Since 1880 the Judith
Mountains have been producing gold and minor amounts of other metals. This group rises
abruptly from the plains in central Montana to an average height of 6000 ft., or about 2000 ft.
above the rolling plains around them. Judith peak, 6386 ft. altitude, is the highest point.
These mountains consist of several detached peaks connected by ridges, and run in general
north-east. Weed and Pirsson first showed (18th Annual Report U. S. G. S. Part 3, Pages 437­616)
that these peaks are the result of laccolithic intrusions in post-Cretaceous time. In the case
of Judith peak, however, no floor of sedimentary rocks, which is necessary in order to have a
laccolith, has been discovered; hence the intrusion is more in the nature of a stock. The outcrop
of this stock is four miles across. More differentiation could occur in this large mass of igneous
rock than in the smaller laccoliths, and therefore the richest gold deposits are found in its vicin­ity.
The laccoliths are chiefly composed of rhyoliteporphyry, but some granite and syenite are
found. Later dikes and sheets of phonolite-porphyry intrude both the rhyolite and sedimentary
rocks. Mineral deposits are found associated with intrusions of both periods. Small amounts of
gold and other metals have been detected, by assay, in the intrusive rocks, and there is no
question but that the mineral deposits were derived from the molten magma.
GEOLOGY. No pre-Cambrian rocks are found in the Judith mountains as in the near-by Little
Rockies and Little Belts, but there occurs a complete and apparently conformable series of
Paleozoic rocks ranging in age from the Cambrian to late Carboniferous. Only the top of the
Cambrian which includes shales, conglomerates, and limestones, is exposed, but this is known
to be over 500 ft. thick. The rest of the Paleozoic is from 1000 to 1200 ft. thick. The
Carboniferous formation in the Judiths is mostly limestone, belonging to the Madison
formation of the Lower Carboniferous. At Yogo in the Little Belts 1200 ft. of shale and
sandstone overlies the Madison and is known as the Quadrant formation, but in the Judiths this
formation appears to be practically absent. Several thousand feet of Jurassic and Cretaceous
shales and sandstones once covered what is now the Judith mountains, but erosion has
removed these except where protected in synclinal folds between two intrusions as at Ruby
Gulch and Maiden. From the tops of the laccoliths erosion has removed from 5000 to 6000 ft.
of sedimentary rocks and exposed the intrusive rhyolite-porphyry. The sedimentary rocks dip
steeply in all directions away from the laccoliths. Some of the peaks are themselves composed
of porphyry, but more often the highest points are ridges of hard limestone, while the valleys
between are eroded in the softer porphyry and Cambrian rocks.
A few small veinlets are found in the porphyry; in Alpine gulch west of Maiden these are gold­bearing
and in Judith peak east of Maiden they carry lead and copper, but they are small and
unimportant. Claims have also been located near the contact of phonolite dikes, but actual gold
production has been small. The valuable orebodies are replacement deposits of gold in lime-

An article published in the Mining and Scientific Press June 10, 1916.

Creator

O. W. Freeman

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newspapers

Type

Text

Date Original

1916-06-10

Subject (keyword)

Judith Mountains, Montana; Geology;

Subject (AAT)

Gold Mines;

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Contributing Institution

Lewistown Public Library, Lewistown, Montana

Publisher (Original)

Lewistown Public Library, Lewistown, Montana

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Judith Mountains, Montana. Central Montana.

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Full text of this item

Gold Mining in the Judith Mountains, Montana
Mining and Scientific Press June 10, 1916
By O. W. Freeman
INTRODUCTION. On the plains of Montana east of the continental divide rise several detached smaller mountain groups. Mineral deposits are found in most of them. Since 1880 the Judith Mountains have been producing gold and minor amounts of other metals. This group rises abruptly from the plains in central Montana to an average height of 6000 ft., or about 2000 ft. above the rolling plains around them. Judith peak, 6386 ft. altitude, is the highest point.
These mountains consist of several detached peaks con¬nected by ridges, and run in general north-east. Weed and Pirsson first showed (18th Annual Report U. S. G. S. Part 3, Pages 437-616) that these peaks are the result of laccolithic intrusions in post-Cretaceous time. In the case of Judith peak, however, no floor of sedimentary rocks, which is necessary in order to have a laccolith, has been discovered; hence the intrusion is more in the nature of a stock. The outcrop of this stock is four miles across. More differentiation could occur in this large mass of igneous rock than in the smaller laccoliths, and therefore the richest gold deposits are found in its vicin-ity. The laccoliths are chiefly composed of rhyolite porphyry, but some granite and syenite are found. Later dikes and sheets of phonolite-porphyry intrude both the rhyolite and sedimentary rocks. Mineral deposits are found associated with intrusions of both periods. Small amounts of gold and other metals have been detected, by assay, in the intrusive rocks, and there is no question but that the mineral deposits were derived from the molten magma.
GEOLOGY. No pre-Cambrian rocks are found in the Judith mountains as in the near-by Little Rockies and Little Belts, but there occurs a complete and apparently conformable series of Paleozoic rocks ranging in age from the Cambrian to late Carboniferous. Only the top of the Cambrian which includes shales, conglomerates, and limestones, is exposed, but this is known to be over 500 ft. thick. The rest of the Paleozoic is from 1000 to 1200 ft. thick. The Carboniferous formation in the Judiths is mostly limestone, belonging to the Madison formation of the Lower Carboniferous. At Yogo in the Little Belts 1200 ft. of shale and sandstone overlies the Madison and is known as the Quadrant formation, but in the Judiths this formation appears to be practically absent. Several thousand feet of Jurassic and Cretaceous shales and sandstones once covered what is now the Judith mountains, but erosion has removed these except where protected in synclinal folds between two intrusions as at Ruby Gulch and Maiden. From the tops of the laccoliths erosion has removed from 5000 to 6000 ft. of sedimentary rocks and exposed the intrusive rhyolite-porphyry. The sedimentary rocks dip steeply in all directions away from the laccoliths. Some of the peaks are themselves composed of porphyry, but more often the highest points are ridges of hard limestone, while the valleys between are eroded in the softer porphyry and Cambrian rocks.
A few small veinlets are found in the porphyry; in Alpine gulch west of Maiden these are gold-bearing and in Judith peak east of Maiden they carry lead and copper, but they are small and unimportant. Claims have also been located near the contact of phonolite dikes, but actual gold production has been small. The valuable orebodies are replacement deposits of gold in limestone near the contact with intrusive laccoliths or sheets. The ore deposits usually occur in well-defined shoots and chimneys, which sometimes consist of high-grade ore surrounded by low-grade. If the ore is near a sheet of porphyry, it is always underneath the porphyry, which serves as the hanging wall. The richest ore, as at the Spotted Horse mine, is accompanied by purple fluorite. The fluorite resulted from the action of hydrofluoric acid on limestone. This acid probably was brought by solu¬tions from the intrusive molten magmas and may have aided in dissolving the gold. The gold-bearing limestone of the ore bodies is usually silicious, often jasperoid, and is more crystalline than before mineralization. The chief mineral deposits occur in the Carboniferous limestone, and are usually restricted to the upper part of it. This probably results from the Upper Carboniferous limestone being thin-bedded with many crevices that mineral-bearing solutions could follow, while the Lower Carboniferous is massive and thick-bedded. The limestone near the intrusions has frequently been brecciated, the angular fragments being cemented and replaced by ore. As the intrusive porphyry cooled, its volume would naturally contract and crevices would open along the contacts with the sedimentary rocks, which would serve for the passage of mineral-bearing solutions. The solutions could more easily dissolve the limestone and replace it with sulphides than the shales and sandstones, therefore little mineralization has occurred except in the limestone. Also carbonaceous matter in the limestone may have helped to precipitate the gold, as would also the lower pressure and temperature in the limestone. The ore bodies are chiefly restricted to limestone and porphyry contacts, but occasionally shoots are found to extend some distance into the limestone, and sometimes the porphyry itself has been replaced. The gold seems to have been chiefly deposited as a telluride along with quartz, pyrite, calcite, and fluorite. Since the mineralization happened, erosion has removed the overlying rocks and exposed the mineral deposits. Surface waters and the air have oxidized the pyrite, chiefly to limonite, resulting in staining much of the ore a red, yellow, or brown. Practically all of the ore thus far mined is softened and decomposed by the agents of weathering. Free gold is rarely seen.
THE MINES in the Judith mountains require little timber, what is required being cut near-by. A good grade of sub-bituminous coal outcrops in the foot-hills a few miles away. Maiden, the chief town in the Judiths, is situated near the centre of the range and has a population of only a 100. At one time nearly 1000 people lived there. Hilger, on a branch railroad from Lewistown, is ten miles from Maiden and is the nearest railroad point. A daily stage connects with the trains. Lewistown, the county seat of Fergus county, is about 18 miles south-west of Maiden. Placer gold was discovered in Alpine gulch near Maiden in 1880, and quartz claims were located soon afterward. The placers were small and were quickly exhausted, but the production from the quartz mines has been probably $9,000,000.
The most important mines are those near Maiden, and of these the best known is the Spotted Horse, which is the largest producer. This mine was located in 1881 and operated fairly continuously until 1902, when it was closed, not to be re-opened until 1912. Its total production is considerably in excess of $5,000,000. Much of the gold produced during its early operation was from high-grade ore, some being worth $30,000 per ton.
The fluoritic gold-ore shipped to the smelter averaged about $100 per ton. A modern cyanide-mill has just been completed and is now in successful operation on the lower-grade ore. The mill has a rated capacity of 50 tons, but its actual capacity is more nearly 75 tons. Before the mill was built, no ore worth less than $20 per ton could be shipped profitably to the smelter, now ore as low as $3 per ton can be milled at a profit. The mill is provided with concentrators, the concentrate being shipped to the smelter. The mine is only developed to a depth of 585 ft., but the geologic conditions are such that it is reasonable to expect large ore bodies. Enough ore is in reserve to supply the mill for several months. The mine is now producing about $12,000 in gold per month.
The Maginnis mine was located about 1880 and was worked at intervals to 1899. It was then closed until 1909, and since then has been worked chiefly by lessees. New machinery is being installed, and the mill is being overhauled. The Maginnis has produced nearly $2,000,000 in gold. The mine is developed to a depth of 426 ft. with over a mile of drifts. It is equipped with a 10-stamp mill, a concentrator for high-grade ore, and a cyanide-plant for the low-grade. As at the Spotted Horse much high-grade ore averaging $100 per ton was shipped to the smelter. The ore of both the Spotted Horse and Maginnis occurs in the Carboniferous limestone below steeply inclined sheets of porphyry that serve as the hanging wall of the stopes. The richest ore, consisting mostly of quartz and fluorite, is of a purple color.
The Cumberland mine, near the Spotted Horse, has been operated in recent years by lessees. It has produced about $270,000, mostly in gold. It is developed by a 1000-ft. adit, and is equipped with a 150-ton cyanide-plant. The ore is found at the contact of limestone and porphyry. Above the adit the known ore has been removed, but recently the lessees have found ore of milling grade below the adit in a drift from a winze 40 ft. deep. Further development is now in progress. The War Eagle is near the Cumberland and since 1910 several shipments of zinc ore carrying gold, silver, and lead have been made. The Cave mine on West Armell's creek has also produced lead-silver ore in recent years. On the north side of the Judiths much development work has been done on the Copper King claim, where copper is the principal mineral. Prospect-ing has also been done on many other claims around Maiden, among which might be mentioned the Mount Grassy group, which has recently been bonded and sampled.
On Elk Mountain at the east end of the range work has been done on a lead-silver claim with encouraging results. The high price of the base metals has encouraged prospecting and work is active on several properties that ordinarily it would not pay to operate.
The Gilt Edge mine is on the east side of the mountains south of Maiden. The town is practically deserted today, but for several years following 1898 the neighboring mines were the most productive of any in the district. Their production is probably not far from $1,500,000. The ore is low-grade, but a high recovery can be obtained cheaply by cyanidation. These mines were located in 1884 and '85. The ore bodies occur under an intrusive sheet of porphyry in the soft thin-bedded Upper Carboniferous limestone. The 300-ton cyanide mill at Gilt Edge produced $1,250,000 in gold from ores that came mostly from the Gold Reef and Great Northern properties. The mill was dismantled a few years ago and the production since then has been small, although a little gold-silver ore has been shipped from time to time.
The Whiskey Gulch mine has a 100-ton cyanide-mill and produced $165,700 in 1903-'04 from $5.50 gold ore, but has been closed since, on account of friction between stockholders. It is claimed that a large amount of low-grade ore is developed. The Mammouth near-by also has a 100-ton cyanide-plant and ore developed that will average $6 per ton, but production has been small and it has been closed for several years owing to disagreement between the owners. A lease has been taken on the Whiskey Gulch mine and it is hoped that both of these properties will be reopened soon.
The New Year mine is on the north side of the range west of Maiden. Several claims were located in New Year gulch in the early '80s, but although much money has been spent on development work the production has been small. As at Gilt Edge the ore bodies occur in limestone under sheets of porphyry. The New Year itself is now idle, development work having failed to disclose any large ore bodies. The New Year mill was re-built in 1911, and is of 150-ton capacity. Development work is now being done on the Gold Acres claims adjoining the New Year property. An adit 800-ft. long has been driven and work is progressing in drifts and raises. It is said that ore worth $4 per ton has been developed. It is planned to treat this ore at the New Year mill, and if development continues favorable to construct a large cyanide-plant. Among the other prospects located near New Year are the claims of Finlaw and Allister, who have found ore suitable for cyanida¬tion.
OUTLOOK. Besides the mines and prospects mentioned, work is being done on others, several of which have produced some gold, such as the old Florence mine. The total production of metals from the district, chiefly in gold, is estimated at from $9,000,000 to $10,000,000. Unfortunately, development has rarely preceded extraction, and usually has been of a haphazard character. Frequently when the ore in sight was exhausted the mine was abandoned. There is little doubt but that careful prospecting and development will disclose large bodies of low-grade cheaply-mined gold ore, and possibly some deposits of the base metals will prove valuable. Mining has been at a low ebb in these mountains for several years, the output in 1914 being only $100,810, mostly in gold. Recently several old properties have been re-opened and other new properties have entered the list of producers. The high price of metals has stimulated prospecting, and the outlook is the best for many years.
SULPHURIC-ACID production of the United States during 1915, expressed in terms of 50°B., amounted .to 3,868,152 short tons, valued at $29,869,080. The aver¬age price was $7.72 per ton, according to the U. S. Geological Survey. The 66° acid sold for $13.95 per ton. By-product acid was as follows:
Source Quantity Value Price Per ton
Copper smelters, 60° 360,522 $2,749,633 $7.63
Zinc smelters 60° 484,942 4,292,493 8.85
Other strengths 59,189 579,115 9.78
________ _________ _____
Total 904,653 $7,621,241 $8.42
60° acid reduced to 50° B 1,056,830
Too much weight must not be attached to the values given in the tables, as prices varied widely during the year. Producers who had previously entered into longtime contracts sold acid at prices much below those now current, especially during the last part of 1915. The trade in strong acids was active on account of the demand from the explosives and war munitions' industries, but this demand came only after the first quarter of the year and was very strong only during the last half. Before that time some acid plants were shut-down. The average values given are therefore much below those which ruled on the market at the close of the year. In 1914 the output of acid, reduced to 50°B., was 3,762,417 tons valued at $24,163,331, or $6.42 per ton. Of this, 950,798 tons was recovered from copper and zinc smelters.

Gold Mining in the Judith Mountains, Montana
Mining and Scientific Press June 10, 1916
By O. W. Freeman
INTRODUCTION. On the plains of Montana east of the continental divide rise several detached
smaller mountain groups. Mineral deposits are found in most of them. Since 1880 the Judith
Mountains have been producing gold and minor amounts of other metals. This group rises
abruptly from the plains in central Montana to an average height of 6000 ft., or about 2000 ft.
above the rolling plains around them. Judith peak, 6386 ft. altitude, is the highest point.
These mountains consist of several detached peaks connected by ridges, and run in general
north-east. Weed and Pirsson first showed (18th Annual Report U. S. G. S. Part 3, Pages 437­616)
that these peaks are the result of laccolithic intrusions in post-Cretaceous time. In the case
of Judith peak, however, no floor of sedimentary rocks, which is necessary in order to have a
laccolith, has been discovered; hence the intrusion is more in the nature of a stock. The outcrop
of this stock is four miles across. More differentiation could occur in this large mass of igneous
rock than in the smaller laccoliths, and therefore the richest gold deposits are found in its vicin­ity.
The laccoliths are chiefly composed of rhyoliteporphyry, but some granite and syenite are
found. Later dikes and sheets of phonolite-porphyry intrude both the rhyolite and sedimentary
rocks. Mineral deposits are found associated with intrusions of both periods. Small amounts of
gold and other metals have been detected, by assay, in the intrusive rocks, and there is no
question but that the mineral deposits were derived from the molten magma.
GEOLOGY. No pre-Cambrian rocks are found in the Judith mountains as in the near-by Little
Rockies and Little Belts, but there occurs a complete and apparently conformable series of
Paleozoic rocks ranging in age from the Cambrian to late Carboniferous. Only the top of the
Cambrian which includes shales, conglomerates, and limestones, is exposed, but this is known
to be over 500 ft. thick. The rest of the Paleozoic is from 1000 to 1200 ft. thick. The
Carboniferous formation in the Judiths is mostly limestone, belonging to the Madison
formation of the Lower Carboniferous. At Yogo in the Little Belts 1200 ft. of shale and
sandstone overlies the Madison and is known as the Quadrant formation, but in the Judiths this
formation appears to be practically absent. Several thousand feet of Jurassic and Cretaceous
shales and sandstones once covered what is now the Judith mountains, but erosion has
removed these except where protected in synclinal folds between two intrusions as at Ruby
Gulch and Maiden. From the tops of the laccoliths erosion has removed from 5000 to 6000 ft.
of sedimentary rocks and exposed the intrusive rhyolite-porphyry. The sedimentary rocks dip
steeply in all directions away from the laccoliths. Some of the peaks are themselves composed
of porphyry, but more often the highest points are ridges of hard limestone, while the valleys
between are eroded in the softer porphyry and Cambrian rocks.
A few small veinlets are found in the porphyry; in Alpine gulch west of Maiden these are gold­bearing
and in Judith peak east of Maiden they carry lead and copper, but they are small and
unimportant. Claims have also been located near the contact of phonolite dikes, but actual gold
production has been small. The valuable orebodies are replacement deposits of gold in lime-